Menu issue

I try to change font-size on the menu, but it also changes size of all other text which I don't want to. I change the code that's on line 255 - 262 on the original style.css but on my childtheme it on other place but the code says:

body,
button,
input,
select,
textarea {
	font-size: 15px; /*SID OCH MENY TEXT*/
	font-size: 1.5rem;
}

I have tried to make a copy of the code and change but it stil changes both. How shall I do? Thanks